I. Technical Field
This invention pertains to the field of solenoids, and particularly to solenoids which operate substantially without audible sound.
II. Related Art and Other Considerations
Most conventional solenoids have two ferromagnetic (e.g., steel or iron) pole pieces, one of which is a moveable pole piece which is attracted to the other (stationary) pole piece upon energization of the solenoid. The moveable pole piece usually comprises or is connected to or integral with a plunger or piston. The moveable piston or plunger, which can be in the form of an output shaft, is the serving or working element/aspect of the solenoid that can be employed in any of various applications or utilizations. See, for example, U.S. Pat. No. 4,812,884 to Mohler, entitled “Three-Dimensional Double Air Gap High Speed Solenoid”, incorporated herein by reference.
Energization of the solenoid is accomplished by applying electrical current to an electromagnetically inductive coil that defines (at least partially) a volume wherein the pole pieces reside. For example, when the coil is energized the two metallic pole pieces can be attracted to one another. The attraction causes an impact of the two pole pieces at the end of travel of the moveable pole piece. Since the two pole pieces are metallic, the impact is noisy. In some applications or environments audible operation of a solenoid is a distraction or worse.
There are also common versions of conventional solenoids which have magnetic latching capability, typically through the use of a magnet in proximity to a pole piece (either in-line or coaxial). See, for example, U.S. patent application Ser. No. 12/109,476, filed Apr. 25, 2008, entitled “ADJUSTABLE MID AIR GAP MAGNETIC LATCHING SOLENOID”, which is incorporated herein by reference.
Some solenoid models attempt to achieve quiet operation by not having metallic pole pieces arranged in-line. In other words, there are no pole pieces arranged to strike or contact each other. For example, a type of solenoid sometimes referred to as a “door bell” solenoid has a coil of wire, wound on a bobbin, with the inner diameter of the coil being the bearing for the plunger. The plunger is essentially a piece of steel. When power is applied to the coil, the plunger is accelerated into the coil. The solenoid frame itself has no stop or base, so that the plunger over travels and hits a ringer, and afterwards bounces back to allow a tone to resonate.
What is needed, and an object of the present invention, are one or more embodiments of solenoids that not only are quiet in plunger operation, but also maintain plunger position after activation without requiring continued application of power.